This invention relates to motor driven hammers, and, in particular to hammers that incorporate an air cushion hammering mechanism.
Such hammers will normally include a tool holder that can hold a hammer bit or chisel bit for acting on a workpiece, and an air cushion hammering mechanism which comprises a piston, a ram and a beat piece that are slidably located in a cylinder so that reciprocation of the piston in the cylinder will be transferred to the beatpiece via the ram and cause the beat piece to repeatedly hit a bit located in the tool holder. Some such hammers may have more than one mode. For example a hammer may be capable of being employed in a hammer only or so-called xe2x80x9cchisellingxe2x80x9d mode in which the piston reciprocates within the cylinder in order to cause the beat piece to hit the bit without any rotation of the tool, or alternatively a drilling only mode in which the cylinder may form part of a spindle connected to the tool holder and is caused to rotate about the piston, thereby causing the bit inserted in the tool holder to rotate. The hammer may also be capable of being employed in a combination rotary hammer mode in which the piston reciprocates within the cylinder causing the beat piece to hit the bit while at the same time the cylinder rotates about its axis, thereby causing the bit to rotate.
One problem with such hammers, when they are hammering a hole into an object, such as a piece of concrete, is that the dust generated by the hammering action can remain in the hole and reduce the efficiency of the hammering action.
According to the present invention there is provided a motor driven hammer which comprises: a hammer housing having a forward end, a cylinder mounted within the hammer housing, a tool holder located at the forward end of the hammer housing forwardly of the cylinder in which tool holder a bit may be releaseably mounted, an air cushion hammering mechanism located in the cylinder for repeatedly impacting a bit mounted in the tool holder, and an eccentric mass, that is an unbalanced mass, mounted for rotation about the axis of the cylinder which can be rotatingly driven so as to cause a bit mounted in the tool holder to be laterally diverted.
The rotation of the eccentric mass will impart a transverse (as opposed to longitudinal) vibration to the hammer and, thus, cause the working end of a bit mounted within the tool holder of the hammer (ie. the end of the bit remote from the tool holder) to tend to follow a circular path. This vibration induced tendency to move or xe2x80x9cwalkxe2x80x9d will be restricted by the structure of the material then being hammered by the bit. The resulting lateral movement of the bit within the material then being hammered is effective to remove dust from the hole and maintain the contact of the tool bit with the unbroken portions of the workpiece. This will improve the performance of the hammer.
In a preferred embodiment of the hammer a sleeve is rotatably mounted on the cylinder which sleeve carries the eccentric mass in an unbalanced manner and can be selectively and rotatingly driven about the cylinder.
The hammering mechanism may comprise a piston, a ram and a beatpiece which are slideably located within the cylinder so that reciprocation of the piston in the cylinder causes the beatpiece to reciprocate and to repeatedly hit a bit located in the tool holder. The hammer may be a rotary hammer that additionally comprises means for causing the cylinder and/or tool holder to rotate in order to cause a bit located in the tool holder to rotate.
In a hammer having a chiselling (hammer only) mode of operation and at least one mode of operation in which the cylinder and/or tool holder is rotatingly driven in order to rotatingly drive a bit mounted in the tool holder, it is preferred that a coupling element is arranged to couple rotary drive to the eccentric mass in the chiselling mode of operation and to decouple rotary drive to the eccentric mass in the modes of operation in which the cylinder and/or tool holder is rotatingly driven.